1. Field of the Invention
The teachings herein relate to seismic tools used in subterranean exploration, an in particular, to techniques for minimizing noise and providing for detection of microseismic events.
2. Background of the Related Art
Subterranean formations may be monitored using one or more seismic receivers. The receivers may be geophones placed at the surface or submerged in wells or on the ocean floor. Also, the receivers may be hydrophones placed in those same locations, but sensitive to only certain types of waves. The receivers placed in wells may be shallow (usually above the formation of interest) or deep (at or below the formation of interest). Seismic receivers may be sensitive to seismic waves along a certain axis or those traveling on any axis. Likewise, the receivers may be sensitive to only certain types of seismic waves, or to several types. Those sensitive to certain axes of travel, called directional receivers, may be coupled with other directional receivers, for example, in a set of three orthogonal receivers which collect information about the waves in three dimensions. This three-dimensional information may be rotated mathematically through the use of trigonometric functions in order to derive information as to wave travel in the x-axis, y-axis, and z-axis relative to gravity. Alternatively, mathematical rotation may provide translation of the data relative to a wellbore, a cardinal direction, or any other reference point.
Microseismic monitoring concerns passively monitoring a formation for seismic events which are very small. Such events may include the seismic effects generated in a formation by fracturing, depletion, flooding, treatment, fault movement, collapse, water breakthrough, compaction or other similar subterranean interventions or effects. One of the main problems with microseismic monitoring, as with other forms of seismic monitoring, is that of noise. With microseismic events, however, the problem is emphasized because the signal strength is generally very small. This means, in turn, that a small amount of noise which would not cause any significant effect as to a regular, active seismic survey causes a significant degradation of the signal to noise ratio in the microseismic survey.
Microseismic surveys include tasks such as receiving data from a receiver, locating data which exceeds some threshold, and analyzing those over-threshold data in order to determine information about certain events. Data which does not meet the threshold may be considered noise data, and may be discarded or simply not recorded.
Microseismic data may be analyzed as a set, with several receivers providing data for a joint analysis. Data is collected from a receiver and related to the other data collected from other receivers in order to derive additional information about the formation. Information from three receivers, for example, may be triangulated in order to estimate the location of a seismic event.
What are needed are a method and a system to make use of seismic data, such as microseismic data previously thought to contain only noise data, in order to derive information about events.